For the purpose of this invention, it will be assumed that the technology of a magnetic compass is well known. The magnetic compass has long been used by navigators to help find direction for steering a vessel. The natural magnetic field of the earth is aligned between the north and south poles closely enough that we have adopted the conventional terms of "magnetic north" and "magnetic south". In reality, however, there is usually some "variation" between true north and south and so-called "magnetic north" and "magnetic south", the latter two terms referring to the directions provided by the needle or orientation of a standard magnetic compass. This "variation" will, at some points on the globe, be minimal. At other points on the globe, however, this "variation" will be quite substantial. A good discussion of magnetic compass navigation is provided in Chapman Piloting, 56th Edition, 1983. The terms described therein, such as "variation" and "deviation" will be adopted for use in this application. What is important to note for the purpose of this discussion is that the relationship between true north and magnetic north is readily ascertainable from geographical and navigational charts and maps.
The earth's natural magnetic field is strong enough to be found at virtually any point on the earth's surface, but is significantly weak to be subject to attenuation by the immediate surroundings of a compass needle, such as metallic objects and electrical currents and conductors. These objects have the effect of disturbing the orientation and intensity of the earth's natural magnetic field. They may cause the magnetic compass reading to be inaccurate. The fluctuations in the reading of a compass needle caused by these fluctuations is called "deviation".
The construction of water vessels, including small pleasure craft with fiberglass hulls, includes a variety of metal components. In particular, it may be said and accepted as well-known, that each vessel has its own unique configuration of such magnetic field disturbances, such as anchor mechanism, radio antenna and sail masts, engine and controls, and appliance and electrical power supply. It may also be said that, once installed, these items are likely to remain in the same configuration for some substantial time, and almost always for the length of a given voyage. (The sail spars will necessarily be moved quite radically to accomplish steering the vessel. When the sail spars are of a magnetic flux material, special consideration must be given to the methods and apparatus described herein which are beyond the scope of this application.)
The combination of metal and electrical components will cause each vessel, including those which are otherwise identical, to have a different electromagnetic flux pattern. For simplicity, this will be referred to as the vessels "electromagnetic signature." Accordingly, the electromagnetic signature of each vessel will cause it to exhibit its own unique relationship between the vessel's actual bearing and that indicated by an on-board magnetic compass.
These variances and deviations will normally be sufficient to adversely affect precise navigation by magnetic azimuth. An error of only five degrees (1/72 of a circle), when maintained over an eleven mile run, will result in a navigator being a full mile off the intended course.
It is generally possible to ascertain the "true bearing" to the sun from any known point on the globe at a given time. Additionally, there is a mathematical formula, called the "Dozier Formula" which enables this direction to be calculated, provided that a person knows their precise coordinates and the precise time. The "Dozier Formula" is:
tan(T-180)=sin LHA/ (cos LHA sin L+cos L tan d) where "T" is the true bearing to the sun, "LHA" refers to the "local hour angle" which is Greenwich mean time plus or minus four minutes for each degree of longitude from the Greenwich meridian, "L" is the latitude, and "d" is the declination angle. It should be noted that while it is true that modern vessels are equipped with sophisticated navigational aids, such as the LORAN (R), it is desirable to have a reliably accurate backup system. There are also those boaters who might find such magnetic navigation either personally preferable or recreational satisfying.
Magnetic compasses, particularly those used for navigation, frequently have adjusting, or compensation, screws. These can be used in order to make a one-time compensation with a compass. Such an adjustment will not, however, be able to account for the deviations to the magnetic reading as a result of the vessel's "electromagnetic signature".
Several devices have been taught in the art to assist with precise navigation using a magnetic compass. Such devices normally include some means of determining a precise direction based upon a known direction to the sun, moon, or other known object.
For instance, U.S. Pat. No. 1,034,767, issued to Cheifetz, Aug. 6, 1912, teaches an apparatus which, by measuring and plotting the angles of shadows from both the sun and the moon, enables a navigator to accurately determine latitude.
Another such device is taught by U.S. Pat. No. 1,570,349, issued to Hollinwood, Jan. 19, 1926. Hollinwood teaches an apparatus which may be used either as a sundial or as a sighting device for a magnetic compass. When used as a sundial, the apparatus depends upon the accuracy of the magnetic direction reading and the sundial face may be tilted to enhance the accuracy of the device.
During recent years, however, the inventive emphasis has been on developing radar and satellite systems to provide precise digital readings of position and direction (along with depth and the location of fish and surface obstacles). It should be noted that modern time-keeping instruments have developed so that relatively inexpensive quartz watches may be relied upon to provide a very precise time. It is also possible to acquire or produce charts which, as a function of date and position, provide accurate information relating such events as sunrise, sunset, and noon meridian to global position or otherwise predict precise solar orientation. Accordingly, it should be possible to develop means and method for refining the accuracy of readings from more traditional magnetic compass instruments.